Libya's deputy ambassador to the UN, Ibrahim al-Dabashi appealed for international intervention, starting with a no-fly zone over the country, to help stop "a real genocide".

Runways at Benghazi airport are reported by Egyptian authorities to have been destroyed in the violence. The country's second city has been the scene of alleged massacres in recent days. The death toll in Libya passed 250 on Monday after six days of unrest, but this is a conservative estimate. The International Federation of Human Rights estimated the death toll at 300 to 400.

Parts of Tripoli were attacked by fighter jets and helicopter gunships overnight. Twenty-six people also died in the eastern city of Al Bayda as it came under fire from forces using aircraft and tanks, according to one eyewitness report.

As both British Airways and bmi cancelled flights from Heathrow to Tripoli, and the Arab carrier Emirates also suspended its services in and out of the capital, the Foreign Office in London was reviewing its advice to Britons without "pressing need to remain" to leave by commercial means, if it is safe to do so. During the protests in Egypt, the UK laid on a charter flight.

"We are monitoring the situation closely. The safety and security of British nationals are our top priority", said a spokeswomanGaddafi appeared briefly on Libyan state TV to deny reports that he had fled the country. "I want to show that I'm in Tripoli and not in Venezuela. Do not believe the channels belonging to stray dogs," he said, holding an umbrella in the rain and leaning out of a vehicle. The station said he was speaking outside his house.

As his forces launched air attacks against protesters amid apparent confirmation of claims that African mercenaries were being used to quell the violence, the UN secretary general, Ban Ki-Moon, condemned the "very disturbing and shocking scenes". He said he had spoken to Gaddafi and "forcefully urged him to stop violence against demonstrators." He told reporters: "This is unacceptable. This must stop immediately. This is a serious violation of international humanitarian law."

The Arab League is also to hold an emergency meeting in Cairo. At least seven Libyan ambassadors have resigned in protest at the killing, although other senior diplomats remained in post while appealing for Gaddafi to step down.

In New York, Dabashi said there must be a no-fly zone "on the cities of Libya so no mercenaries, no supplies of arms will arrive to the regime". He told a press conference he and other UN diplomats were not resigning because they served the people of Libya, not the regime.

"This is in fact a declaration of war against the Libyan people," he told reporters, surrounded by a dozen Libyan diplomats. "The regime of Gaddafi has already started the genocide against the Libyan people."

Libya's ambassador to the United States, Ali Aujali, called for Gaddafi to step down. "There's no other solution. He should step down and give the chance for the people to make their future," he told Associated Press. "How can I support the government killing our people? … What I have seen in front of my eyes now is not acceptable at all."

Aujali said he was not resigning his post, because he was on the "good side" of the Libyan government and not part of the killing. "There are many people working very hard to make things work in the right way but, unfortunately, we don't have enough power that we can change everything going on in Libya," he said.

Libya's ambassador at the UN, Abdurrahman Mohamed Shalgham, told the pan-Arab newspaper, al-Hayat, that all diplomats at Libya's mission supported Dabashi, "excluding me". Shalgham said he was in touch with the Gaddafi government and was trying "to persuade them to stop these acts".

The country's ambassador to India, who resigned over the crackdown, said African mercenaries were being employed by Gaddafi. "They are from Africa, and speak French and other languages," Ali al-Essawi told Reuters, adding he had been told there had been army defections.

"They [the troops] are Libyans and they cannot see foreigners killing Libyans so they moved beside the people."

In Al Bayda, resident Marai Al Mahry told Reuters by telephone that 26 people, including his brother Ahmed, had been shot dead overnight by Gaddafi loyalists. "They shoot you just for walking on the street," he said.

"The only thing we can do now is not give up: no surrender, no going back. We will die anyway, whether we like it or not. It is clear that they don't care whether we live or not."